(1) Field of the Invention
The present invention relates to a cationic electrodeposition coating composition having a low volatile organic solvent content that is, having a low content of a volatile organic compound (hereinafter may be referred to as VOC).
(2) Description of Background Art
The cationic electrodeposition coating composition has a high throwing power and is capable of forming a coating film showing good properties in durability, corrosion resistance and the like, and has widely been used as a primer coating composition coated on an electrically conductive metal such as an automobile body, electrical All articles and the like, in which the above coating film properties are required.
The electrodeposition coating composition in the prior art needs to contain 2% by weight or more of a volatile organic solvent such as monoalcohols, glycols, cellosolves, ketones and the like, so that a satisfactory film-forming properties may be maintained. On the other hand, demand on reducing a content of a volatile component in the electrodeposition coating composition, for example, an organic solvent from the standpoints of prevention of environmental disruption, saving of resources and the like has been increased.
In view of the above, it may be proposed to simply remove the volatile component in the electrodeposition coating composition so that a content of the volatile component may be 1% by weight or less, resulting in that reduction in film-forming properties makes it difficult to form a cured coating film having a film thickness of 15 xcexcm or more, and reduces smoothness and corrosion resistance. On the other hand, in the application of the electrodeposition coating composition to a galvanized alloy, for example, an automobile body as widely used as a substrate, a high energized voltage for the purpose of thickening the electrodeposition coating film develops pinholes due to generation of gas, resulting in undesirably reducing smoothness of the coating film.
It is an object of the present invention to provide a cationic electrodeposition coating composition having a volatile organic solvent content in the range of 1% by weight or less without reducing throwing power, corrosion resistance, etc. and showing good film-forming properties, and is capable of forming a coating film showing good smoothness.
It is another object of the present invention to provide a cationic electrodeposition coating composition capable of applying to a galvanized alloy to form a thick coating film having a film thickness of 15 xcexcm or more without developing pinholes due to generation of gas.
That is, the present invention provides a cationic electrodeposition coating composition comprising (A) a cationic resin having a crosslinkable functional group and a cationic group in one molecule, (B) a crosslinking agent, and (C) polyether polyol, said cationic electrodeposition coating composition having a volatile organic solvent content in the range of 1% by weight or less.
The cationic electrodeposition coating composition of the present invention may include a water-based coating composition which essentially contains a cationic resin (A) having a crosslinkable functional group such as hydroxyl group and a cationic group in one molecule, a crosslinking agent (B) and polyether polyol (C), and optionally contains color pigments, anti-corrosive pigments, extender pigments, organic solvents, acids for use in neutralization, anti-cissing agents, surface active agents, catalysts, inhibitors, rheology-controling agents, pigment dispersants and the like.
The cationic resin (A) has a crosslinkable functional group such as hydroxyl group and a cationic group in one molecule, and may include, as a resin skelton, epoxy resin, acrylic resin, polybutadiene, alkyd resin, polyester resin and the like. Of these, a cationic resin prepared by addition of an amine to the epoxy resin is preferable from the standpoint of good corrosion resistance.
Examples of the cationic resin prepared by addition of the amine to the epoxy resin may include (1) adducts of an epoxy compound with primary mono or polyamine, secondary mono or polyamine, primary and secondary mixed polyamine or the like (see U.S. Pat. No. 3984299 Specification); (2) adducts of an epoxy compound with a ketiminized, primary amino group-containing mono or polyamine (see U.S. Pat. No. 4017438 Specification); (3) reaction products obtained by etherification between an epoxy compound and a ketiminized, primary amino group-containing hydroxyl compound (see Japanese Patent Application Laid-Open No. 43013/84), and the like. These may be modified with a modifier such as polyamide resin and the like.
The above epoxy compound has at least two epoxy groups in one molecule and a number average molecular weight in the range of 400 to 4000, and may be prepared by a reaction between a polyphenol compound and epichlorohydrin. Examples of the polyphenol compound may include bis (4-hydroxyphenyl)2,2-propane, 4,4-dihydroxybenzophenone, bis(4-hydroxyphenyl)-1,1-ethane, bis(4-hydroxyphenyl)-1,1-isobutane, bis(4-hydroxy-tert-butylphenyl)-2,2-propane, bis(2-hydroxynaphthyl)methane, 1,5-dihydroxynaphthalene, bis(2,4-dihydroxyphenyl)methane, tetra(4-hydroxyphenyl)-1,1,2,2-ethane, 4,4-dihydroxyphenylsulfone, phenol novolak, cresol novolak, and the like.
The crosslinking agent (B) may include known crosslinking agents such as blocked polyisocyanate compound, amino resin and the like. The blocked polyisocyanate compound may include a reaction product obtained by subjecting all of isocyanate groups to addition reaction of a blocking agent to the isocyanate groups. The polyisocyanate compound is a compound having at least two isocyanate groups in one molecule, and may include known aromatic, alicyclic and aliphatic polyisocyanate compounds, and isocyanate group-terminated prepolymers obtained by reacting a low molecular weight active hydrogen-containing compound such as glycol or triol with an excess amount of the above polyisocyante compounds. Examples of the blocking agent may include lactams, oximes, phenols, aliphatic alcohols, aromatic alkyl alcohols, ether alcohols, and the like.
A mixing ratio of the cationic resin (A) to the crosslinking agent (B) is such that the cationic resin (A) is in the range of 50 to 90% by weight, preferably 65 to 80% by weight, and the crosslinking agent (B) is in the range of 50 to 10% by weight, preferably 35 to 20% by weight based on a total weight of solid contents of both components (A) and (B) respectively.
The cationic resin (A) may be neutralized with a water-soluble organic acid such as formic acid, acetic acid, lactic acid and the like so as to be water-soluble or water-dispersible.
Examples of the anti-corrosive pigment may include zinc dust, zinc phosphate, calcium phosphate, aluminum phosphate, aluminum polyphosphate such as aluminum tripolyphosphate, aluminum orthophosphate, calcium orthophosphate, boric acid based anti-corrosive pigment, alkaline earth metal, zinc oxide and the like, compound oxides of iron oxides with at least one of the above compounds, tungstic acid based anti-corrosive pigments, phosphite based anti-corrosive pigments, hypophosphous acid based anti-corrosive pigments, nitrite based anti-corrosive pigments, vanadic acid based anti-corrosive pigments, zinc formate, zinc acetate, zinc octenate and the like, and further may include compounds represented by chemical formulas such as Zr(OH)4, Mg4Al2(OH)12CO3xc2x73H2O, Mg6Al2(OH)16CO3xc2x75H2O and Mg6Al7(OH)16CO3xc2x74H2O; and bismuth-containing compounds such as bismuth lactate, bismuth silicate, triphenylbismuth, bismuth gallate, bismuth hydroxide, bismuth trioxide, bismuth nitrate, bismuth benzoate, bismuth citrate, bismuth oxycarbonate, and the like.
The above anti-corrosive pigments may usually be used as a powder having a particle size of 10 xcexcm or less, particularly 0.01 to 3 xcexcm, and may suitably be used in the range of 0. 5 to 100 parts by weight, particularly 1 to 50 parts by weight per 100 parts by weight of a total resin solid content of the cationic resin (A) and crosslinking agent (B) used in the cationic electrodeposition coating composition.
Examples of the polyether polyol (C) may include alkylene based polyether polyol such as polyoxymethylene glycol, polyoxyethylene glycol, polypropylene glycol, polyoxybutylene glycol and the like; aromatic ring-containing polyether polyol such as bisphenol alone and combination of bisphenol with glycol; and the like. The above polyether polyol suitably has a number average molecular weight of 200 to 3000, preferably 600 to 1500, and a boiling point of 200xc2x0 C. or tea higher. The above polyether polyol (C) does not include the volatile organic solvent.
The polyether polyol (C) may be contained in the range of 0.5 to 15 parts by weight, particularly 2 to 10 parts by weight per 100 parts by weight of a total resin solid content of the cationic resin (A) and the crosslinking agent (B).
The cationic electrodeposition coating composition of the present invention may be prepared, for example, by a process which comprises adding a cationic resin (A) having a crosslinkable functional group and a cationic group in one molecule and a crosslinking agent (B) into a volatile organic solvent, adding thereinto polyether polyol (C), followed by adding an acid to neutralize the cationic resin, dispersing into water to form an emulsion, uniformly mixing, and vacuuming and removing the volatile organic solvent contained in the emulsion.
The volatile organic solvent may preferably has a boiling point of about 130xc2x0 C. or lower, preferably 120xc2x0 C. or lower, so that all or almost all of the organic solvent contained in the emulsion may be removed during the vacuuming step. Examples of the organic solvent having the above boiling points may include methyl ethyl ketone, mehyl isobutyl ketone, isopropanol and the like. Use of the above volatile organic solvent makes it possible to easily control a volatile organic solvent content in the electrodeposition coating bath on electrodeposition coating in the range of 1% by weight or less. The vacuuming step may preferably be carried out under a vacuum of xe2x88x92700 mm Hg or lower.
The cationic electrodeposition coating composition prepared as above containing the component (C) has a volatile organic solvent content of 1% by weight or less, preferably 0.5% by weight or less and is coated under known electrodeposition coating conditions, resulting in showing good film-forming properties without reducing throwing power and corrosion resistance, and in making it possible to obtain a coating film showing good smoothness, particularly to coat so as to form a thick cured coating film of 15 xcexcm or more, preferably 20 xcexcm or more even if coated onto a galvanized alloy without developing pinholes due to generation of gas. That is, use of the above cationic electrodeposition coating composition makes it possible to reduce a volatile organic solvent content to 1% by weight or less without reducing corrosion resistance and smoothness, resulting in achieving an object of reducing the volatile organic compound (VOC).
The cationic electrodeposition coating composition of the present invention is diluted with deionized water or the like so as to be a solid content of about 5 to 40% by weight and controlled so as to form an electrodeposition coating bath having a pH of 5.5 to 9.0. A coating object as a cathode is dipped into the electrodeposition coating bath to be energized under the conditions of a bath temperature of 15 to 35xc2x0 C. and a loading voltage of 100 to 400V to carry out the electrodeposition coating. Even if coated onto a galvanized alloy, a cured coating film having a thickness of 15 xcexcm or more may easily be formed. After the completion of energizing, the coating object may be taken out of the electrodeposition coating bath, followed by washing with water, and heating at 100 to 200xc2x0 C. for 10 to 40 minutes to cure the coating film.
The cationic electrodeposition coating composition containing the polyether polyol has a volatile organic solvent content of 1% by weight or less and is coated under known electrodeposition coating conditions, resulting in showing good film-forming properties without reducing throwing power and corrosion resistance, and in making it possible to obtain a coating film showing good smoothness, particularly to coat so as to form a thick cured coating film of 15 xcexcm or more, preferably 20 xcexcm or more even if coated onto a galvanized sheet metal without developing pinholes due to generation of gas. That is, the present invention makes it possible to provide the cationic electrodeposition coating composition having a volatile organic solvent content of 1% by weight or less without reducing corrosion resistance and smoothness, resulting in achieving an object of reducing the volatile organic compound (VOC).